PHYSICAL REVIEW B 99, 134520 (2019)

Surface terminations and layer-resolved tunneling spectroscopyof the 122 iron pnictide superconductors

Ang Li,1,* Jia-Xin Yin,2 Jihui Wang,1 Zheng Wu,1 Jihua Ma,1,3 Athena S. Sefat,4 Brian C. Sales,4 David G. Mandrus,4

Michael A. McGuire,4 Rongying Jin,5 Chenglin Zhang,6 Pengcheng Dai,6 Bing Lv,1,† Ching-Wu Chu,1 Xuejin Liang,2

P.-H. Hor,1 C.-S. Ting,1 and Shuheng H. Pan1,2,7,8,9,‡

1Department of Physics and Texas Center for Superconductivity, University of Houston, Houston, Texas 77204, USA2Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

3Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467, USA4Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA

5Department of Physics & Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA6Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA

7School of Physics, University of Chinese Academy of Sciences, Beijing 100190, China8CAS Center for Excellence in Topological Quantum Computation, University of Chinese Academy of Sciences, Beijing 100190, China

9Songshang Lake Material Laboratory, Dongguan, Guangdong 523808, China

(Received 25 April 2017; revised manuscript received 28 March 2019; published 30 April 2019)

The surface terminations of 122-type alkaline earth metal iron pnictides AEFe2As2 (AE = Ca, Ba) areinvestigated with scanning tunneling microscopy/spectroscopy. Cleaving these crystals at a cryogenic temper-ature yields a large majority of terminations with an atomically resolved (�2 × �2)R45 or 1 × 2 lattice, aswell as a very rare termination of 1 × 1 lattice symmetry. By analyzing the lattice registration and selectivechemical marking, we identify these terminations as (�2 × �2)R45-reconstructed AE, 1 × 2-reconstructed As,and (�2 × �2)R45-reconstructed Fe surface layers, respectively. Layer-resolved tunneling spectroscopy onthese terminating surfaces reveals a well-defined superconducting energy gap on the As terminations, while thegap features become weaker on the AE terminations and absent on the Fe terminations. The superconductinggap is hardly affected locally by the As or AE surface reconstructions. The definitive identification of thesurface terminations and the associated spectroscopic signatures shed light on the essential roles of As andthe pnictogen-iron-pnictogen trilayer building block in iron-based superconductivity.

DOI: 10.1103/PhysRevB.99.134520

I. INTRODUCTION

The discovery of iron-based superconductors marked sig-nificant progress in the study of high-temperature super-conductivity [1–3]. All iron-based superconductors share aunique pnictogen (chalcogen)-iron-pnictogen (chalcogen) tri-layer structural unit, and it is generally believed that super-conductivity develops primarily in the iron plane with 3dorbital characters [4]. To unravel the multiorbital nature ofthe iron plane and to understand the roles of other ingredientlayers, an atomic layer-resolved electronic characterization isthus essential. Scanning tunneling microscopy/spectroscopy(STM/STS) is an ideal tool for extracting local structural andspectroscopic information. It has played a critical role in ex-ploring the electronic interactions in iron-based superconduc-tors [5]. So far, iron-based superconductors like Fe(Se,Te) andLiFeAs have been relatively well characterized by STM/STS

*Corresponding author: angli@mail.sim.ac.cn; current address:State Key Laboratory of Functional Materials for Informatics,Shanghai Institute of Microsystem and Information Technology,Chinese Academy of Sciences, Shanghai 200050, China.

†Current address: Department of Physics, The University of Texasat Dallas, Richardson, Texas 75080, USA.

‡Corresponding author: span@iphy.ac.cn

due to their definitive crystal cleavage, while the studyof 122-type alkaline earth metal iron pnictides AEFe2As2(AE = Ca, Ba, etc.) suffers from controversial identificationsof the cleavage planes due to the complex surface mor-phologies [6–23]. Therefore, clarification of the cleavage,identification of the resulting terminations, and subsequentlayer-resolved spectroscopic investigations become crucialand highly demanded for studying the AE122 iron-basedsuperconductivity.

Among iron-based superconductors, the AE122 com-pounds are noteworthy for their relatively high TC and widelyaccessible chemical doping range [2,3]. They have a lay-ered crystal structure with the weakest bonding between theadjacent AE and As layers [Fig. 1(a)]. Accordingly, thesecrystals tend to cleave there between and the AE and As planesare most likely to be exposed as the surface terminations.As the adjacent As layer and AE layer are ionically bondedin bulk, upon cleaving, the unbalanced charge distributionon the terminations can lead to various surface reconstruc-tions making the surface identification more complicated.Based on the early STM studies [6–23], two major assign-ments have been proposed for the AE122-crystal surfaceterminations: (1) Upon cleaving, the AE atoms are dividedstatically into two 50% portions that redistribute uniformlyonto the two adjacent As terminating surfaces to form ei-ther a (�2 × �2)R45 or a 1 × 2 superstructure as a 50%

2469-9950/2019/99(13)/134520(6) 134520-1 ©2019 American Physical Society

ANG LI et al. PHYSICAL REVIEW B 99, 134520 (2019)

FIG. 1. (a) Schematic crystal structure of AEFe2As2. (b)–(d)Surface morphologies demonstrating (b) 1 × 2 (V = 100 mV, I =30 pA), (c) rt2 (V = 20 mV, I = 0.2 nA), and (d) 1 × 1 (V = 50 mV,I = 2 nA) superstructures. Insets in (b)–(d) are the zoom-in imageof 1 × 2 dimers (V = 20 mV, I = 8 nA), rt2 buckling (V = 20 mV,I = 2 nA) and 1 × 1 surface (V = 50 mV, I = 2 nA), respectively.

AE coverage [6–9,12,14,19–23]; (2) the AE atomic layer isdisassembled into scattered atoms, clusters, or lumps insteadof remaining in a complete atomic coverage layer, then theAs layer is exposed with (�2 × �2)R45 or 1 × 2 reconstruc-tion [10,11,13,18]. In this article we present high-resolutionSTM/STS results on the cryogenically cleaved AEFe2As2[Ba(Fe1−xCox )2As2, Ba1−xKxFe2As2, and CaFe2As2] singlecrystals. We demonstrate that three types of ordered surfacestructure can be distinguished as (1) (�2 × �2)R45 recon-struction of the complete AE lattice, (2) 1 × 2 stripes from theAs dimerization, and (3) the rarely observed (�2 × �2)R45pattern of the Fe lattice. Local differential conductance mea-surements show superconducting gap features in the low-energy excitation spectra on both AE and As terminations,while not in that on the Fe exposure. Such spectroscopicdiscrepancies indicate that the As-Fe-As trilayer structure isessential to the superconducting pairing in iron pnictides.

II. EXPERIMENTAL METHOD

The single crystalline samples of BaFe2As2 and CaFe2As2

with various doping were grown using the self-flux method[24–26]. In particular, the CaFe2As2 crystals were furnace-cooled to room temperature without quenching, thus no traceof a low-temperature collapsed tetragonal phase was detectedin the electrical resistivity and magnetic susceptibility mea-surements [27]. The STM/STS experiments were carried outon a home-built ultrahigh vacuum low-temperature STM.Samples were cleaved in situ below 30 K and immediatelytransferred to the STM head, which was already at the base

FIG. 2. (a) The step height of ∼5.6 Å between two stripe sur-faces, which is consistent with the crystallographic dimension of ahalf unit cell c/2 = 5.8 Å. (b) The step between a rt2 surface and astripe surface with the apparent step height of ∼0.12 Å, which is oneorder of magnitude too small than any of the crystallographic spacingin this crystal. For both cases, the topological profile along the linemarked in the upper topographic image is shown in the lower panel.

Data are both taken on CaFe2As2 at 4.2 K for an area of 300×300 Å2

with tunneling junction set up: V = 50 mV, I = 0.5 nA.

temperature of 4.3 K. The scan tips were prepared frompolycrystalline tungsten wires by electrochemical etching andsubsequent field-emission cleaning. Topographic images wereacquired in the constant-current mode with the bias volt-age applied to the sample. Differential conductance spectrawere recorded with the standard lock-in technique. Whendescribing the lattice symmetry, we choose the ThCr2Si2-typetetragonal notation throughout this article so that the low-temperature orthorhombic symmetry [28] can be denoted as“(�2 × �2)R45”, or “rt2” for short.

III. RESULTS AND DISCUSSION

The most commonly (about 99% of the time) observedterminating surfaces of an AEFe2As2 single crystal resultedfrom cold cleaving are shown in Figs. 1(b) and 1(c). The 1 × 2superstructure in Fig. 1(b) consists of one-dimensional stripeswith interstripe distance ∼8 Å, twice the tetragonal latticeconstant. Along the stripe are grains at ∼4 Å spacing andwithin each grain there are two atoms that can be resolvedto form a dimer in high-resolution images [inset in Fig. 1(b)].This dimerization can follow either the a or b direction, thustwin domain structures are often observed on a striped surfaceas shown in Fig. 1(b). The dimerization is most likely a surfacephenomenon for no evidence has been reported from bulkmeasurements such as x-ray diffraction and high-resolutiontransmission electron microscope. The structural nature of the1 × 2 symmetry is further evidenced by the independence oftopographic image on the bias voltage. The second type oftermination shown in Fig. 1(c) exhibits a bias-independentsquare-like lattice with a much smaller topographic corruga-tion. There, the unit cell is enlarged by �2 × �2 times fromthe tetragonal 1 × 1 base and it seems that only 50% of theatoms in the atomic layer are resolved if one attempts to assignthem to the AE or As layer. High-resolution imaging, however,

134520-2

SURFACE TERMINATIONS AND LAYER-RESOLVED … PHYSICAL REVIEW B 99, 134520 (2019)

FIG. 3. (a) The joint area between rt2 and 1 × 2 in CaFe2As2 (400×400 Å2, V = 50 mV, I = 1 nA). (b) The zoom-in images of boundaries

as marked in the red box in (a). (c) Schematic drawings for the in-plane atomic lattice registration at joint boundary of 1 × 2 and rt2 areas whenthey belong to two different atomic layers (As and AE). Ellipses and solid lines represent the dimers and rt2 superlattice respectively. The rt2

topographies of Ba0.6K0.4Fe2As2 are shown in (d) 500×500 Å2, V = −100 mV, I = 2 nA, and (e) 125×125 Å

2, V = 100 mV, I = 2 nA. (f)

1 × 1 area surrounded by the 1 × 2 stripes in CaFe2As2 (130×130 Å2, V = 50 mV, I = 1 nA). (g) Schematic drawings for the in-plane atomic

arrangement of stripes vs 1 × 1 lattice assuming that 1 × 1 is rt2-buckled Fe (As: dark blue, Fe: dark and light red), and the height profilesalong the two orthogonal directions. (h) Height profiles along the 1 × 1 lattice in orthogonal directions indicated in (f). A half-unit-cell latticeshift in the 1 × 1 profile relative to the stripe profile can be clearly observed.

reveals the complete atomic coverage with a �2 × �2 buck-ling reconstruction [inset in Fig. 1(c)]. Such a rt2 lattice recon-struction is consistently observed even in heavily overdopedBa(Fe1−xCox )2As2 samples, where the bulk low-temperatureorthorhombic/magnetic phase is completely suppressed [29].Therefore, we conclude that the buckled rt2 superstructure isa surface phenomenon as well. In addition to 1 × 2 and rt2, athird type of surface morphology with 1 × 1 lattice symmetry[Fig. 1(d)] is very rarely observed. Knowing the easy cleavageplane lies between the AE and As layers, we expect thecommonly observed 1 × 2 and rt2 surfaces are associated withthe AE and/or As terminations exposed after cold cleaving.

As the AE and As layers share the same crystal latticesymmetry in the bulk, it is not decisive to assign the atomicidentity of the 1 × 2 or rt2 reconstructed surface merely basedon the structural symmetry. Due to the fact that the constantcurrent topographic image convolutes the spatial variation ofthe integrated local density of states (LDOS) and the geo-metrical corrugations, simply assigning these terminations bytheir apparent image height would not be appropriate either.As an example, on a cleaved CaFe2As2 crystal, while theapparent step height between two stripe surfaces is consistentwith the crystallographic dimension of a half unit cell c/2[Fig. 2(a)], the apparent step height between a rt2 surface anda stripe surface in its STM image is one order of magnitudetoo small than any of the crystallographic spacings in the bulk

crystal [Fig. 2(b)]. This clearly demonstrates that it is impossi-ble to rely on the apparent step height in determination of theidentities of two surfaces with different chemical identities orsuperstructures, not even to determine whether they are twodifferent elemental terminations or simply the same elementaltermination but with different surface reconstructions.

After carefully examining these topographic images andcomparing with the crystal structures, we have developed a setof practical schemes to help in identifying these terminations.First, we apply the relative lattice registration scheme andhave noticed a critical clue from the in-plane lattice alignmentbetween the adjacent 1 × 2 and rt2 regions in Fig. 3(a). Thezoom-in image shown in Fig. 3(b) clearly demonstrates thatthe centerline of each stripe points in 45°exactly to the rt2superlattice grid, indicating the two atomic lattices’ half-unit-cell alignment. This observation supports the fact that the twocommonly observed surface structures are not from the sameatomic plane, but are associated with AE and As terminations,respectively, as shown in the schematic in Fig. 3(c). Second,we apply the selective doping scheme to determine theircorrespondence. As an example, we chose potassium to dopeBaFe2As2, in which potassium dopants partially substitutethe Ba atoms [20]. In its rt2 topographic images [Fig. 3(d)and 3(e)], bright and dark sites are readily visible at atomiclevel, while such a contrast is absent in the 1 × 2 stripes. Thepercentage of dark sites is approximately 40%, and agrees

134520-3

ANG LI et al. PHYSICAL REVIEW B 99, 134520 (2019)

fairly well with the nominal concentration of K doping. Thisleads us to conclude that the two major surface terminationsare created by cold cleaving between AE and As planes;the exposed AE layer buckles to form a rt2-superstructureand the As atoms in the arsenic plane dimerize to formone-dimensional stripes. Each of the two terminations alonedoes not cover the entire cleaving surface. Instead, the entiresurface is covered by a roughly 50–50% mixture of them thatare separated by single-atom steps [Fig. 2(b)]. The apparentstep height of such single-atom step in the STM topographicimages is much smaller (∼0.7 Å for BaFe2As2 and ∼0.1 Åfor CaFe2As2) as compared to the crystalline AE-As interlayerdistance (1.9 Å [28]), which can be explained by AE’s smallercontribution to the density of states near the Fermi level [30].

Having identified the commonly observed rt2 and 1 × 2terminations, we then turn to the rare 1 × 1 termination[Figs. 1(d) and 3(f)]. Considering its very low probability ofoccurrence, the 1 × 1 termination could be either an unre-constructed version of the AE or As exposure [22,31] or the(�2 × �2)R45 reconstructed pattern of the Fe lattice (whoseoriginal lattice constant is 1/�2 times of that in As and AE).Again, we apply the lattice registration scheme. Note thatthe rt2 pattern of Fe, when surrounded by As stripes, shouldhave a unique atomic registration with the As stripes fromtwo orthogonal directions as illustrated in Fig. 3(g). There isa half-unit-cell shift in the alignment of rt2 Fe lattice withrespect to the As stripes from the a and b directions. Weemphasize that this atomic lattice registration is exclusive forFe terminations with rt2 reconstruction. The height profilesalong the two lines in Fig. 3(f) clearly demonstrate such alattice registration with a half-unit-cell shift as highlightedin Fig. 3(h), which provides decisive evidence of the 1 × 1structure being the rt2 reconstructed pattern of the unusuallyexposed Fe layer.

The combination of low temperature cleaving and thedefinitive identification of all building layers provide one anextraordinary opportunity to perform layer-resolved spectro-scopic investigations of AEFe2As2. In Fig. 4, the spatiallyaveraged differential conductance spectra on optimally dopedBa(Fe, Co)2As2 (TC ≈ 22 K) show the superconducting en-ergy gap on both Ba and As terminations with the gap mag-nitude � ≈ 6 meV and the ratio 2�/kBTC ≈ 6.3, indicatinga strong coupling superconducting state. Interestingly, thegap features are well defined in the 1 × 2 As surface, whilethe spectrum on Ba termination exhibits reduced coherencepeaks and more in-gap states near the Fermi level, probablycaused by impurity scattering effect of Co doping. The moregeneral discussion of the spectral differences from these twoterminations is still open, and we present our view from theorbital perspective [32].

In strong contrast, the Fe termination is characterized byan overall V-shaped spectrum without features of supercon-ductivity. This is quite puzzling as that even if there is nocoherent Cooper pairing in the exposed Fe layer due to themissing As coverage, there should still be a superconductinggap feature from the proximity of superconducting bulk lyingbelow. A possible scenario is that a metallic state coexists inthe region (caused by the surface termination/reconstruction)and obscures the observation of the proximity induced super-conducting gap. If this is the case, then why this metallic state

FIG. 4. Spatially averaged differential conductance spectra onthree surface terminations in optimally doped Ba(Fe, Co)2As2 (V =−20 mV, I = 0.67 nA). The insets show the corresponding topo-

graphic images with the size of 35×35 Å2.

is immune to the proximity effect needs an explanation. Apossibility along another line is that magnetism can suppressthe phase coherence of Cooper pairs [33], thus it is conceiv-able that without coverage of As, the Fe-terminated surfacepossesses strong magnetism that destroys the superconductingphase coherence. If this is the case, one should still see a gap-like DOS depression without coherent peaks in the tunnelingspectrum, but this feature is absent. In any case, the V-shapedspectrum without any superconducting gap features on theexposed Fe surface compared with the coherent gap spectrumon the As surface essentially highlights the role of the Aslayer and the vital integrity of the As-Fe-As trilayered block inthe electronic pairing and the emergence of superconductivity.Indeed, the magnetic and electronic properties depend verysensitively on the Fe-As local structures [34,35]. Removingthe As layer can dramatically alter the coordination environ-ment of Fe, hence the pairing of electrons.

In addition to the chemical identity, the lattice reconstruc-tion can also influence the surface electronic structure byinducing band folding in the momentum space. Our previousphotoemission measurement shows that the 1 × 2 surface re-construction leads to a noticeable band folding, with the bandaround the � and M point of the Brillion zone being foldedto the X position [36]. In our STM/S observations, the rep-resentative broken symmetry in the AE and As terminations,however, does not alter the low-energy local DOS spectrumsignificantly. For example, no perceivable change in the gapsize is detected with 1 × 2 periodicity [Figs. 5(a) and 5(b)],

134520-4

SURFACE TERMINATIONS AND LAYER-RESOLVED … PHYSICAL REVIEW B 99, 134520 (2019)

FIG. 5. (a) and (b) Series of differential conductance spectrameasured along and across the As stripes for optimally dopedBa(Fe, Co)2As2 (V = −20 mV and I = 0.67 nA). The trajectoriesare drawn in the inset of (a). Spectra are offset for clarity. The insetimage shows the corresponding topographic image with the size of

25×50 Å2. (c) and (d) Intensity plot of the spectra in (a) and (b),

respectively.

except that the width of the coherence peaks is very weaklymodulated across the stripe [Fig. 5(d)]. This is consistent withthe fact that the superconducting coherence length (typicallymore than 20 Å [7,16]) is much larger than the periodicities ofthese superlattices.

IV. SUMMARY

We have performed an STM/STS study on the AE-122 ironpnictides. Cleaving at a cryogenic temperature exposes thepredominant rt2-buckled AE termination and 1 × 2-dimerizedAs termination. Very occasionally the crystal cleaves betweenthe As and Fe layers leaving the rt2 reconstructed pattern ofthe Fe layer exposed with characteristic 1 × 1 lattice symme-try. The superconducting energy gap is observed on AE andAs terminations, while no gap features are found on the Fetermination. Our atomic layer-resolved spectroscopic studysuggests that the As-Fe-As trilayer block is essential for thesuperconductivity in 122 pnictides. The definite identificationof various terminating surfaces and detailed spectroscopiccharacterization on each termination provide us valuable in-formation towards a comprehensive understanding of the iron-based superconductivity. Our methodology of atomic surfaceidentification and layer-revolved tunneling demonstrated herecan also be generally applied to study other complex layeredquantum materials [37,38].

ACKNOWLEDGMENTS

We sincerely thank Dr. D. Lu and Dr. Z. Y. Lufor helpful discussions. This work is supported by theChinese Academy of Sciences, NSFC (11227903), BM-STC (Z181100004218007), the Ministry of Science andTechnology of China (2015CB921300, 2015CB921304,2017YFA0302903), the Strategic Priority Research ProgramB (XDB04040300, XDB07000000). A.L. acknowledges sup-port from the Shanghai Science and Technology Committee(18ZR1447300). Work at University of Houston was sup-ported by the State of Texas through TcSUH. C.-S.T. acknowl-edges support of the Robert A. Welch Foundation (E-1146).Work at ORNL was supported by the U. S. Department ofEnergy, Office of Science, Basic Energy Sciences, MaterialsSciences and Engineering Division.

[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.Chem. Soc. 130, 3296 (2008).

[2] D. Johnston, Adv. Phys. 59, 803 (2010).[3] G. R. Stewart, Rev. Mod. Phys. 83, 1589 (2011).[4] K. Juroki, S. Onari, R. Arita, H. Usui, Y. Tanaka, H. Kontani,

and H. Aoki, New J. Phys. 11, 025017 (2009).[5] J. E. Hoffman, Rep. Prog. Phys. 74, 124513 (2011).[6] M. C. Boyer, K. Chatterjee, W. D. Wise, G. F. Chen, J. L. Luo,

N. L. Wang, and E. W. Hudson, arXiv:0806.4400.[7] Y. Yin, M. Zech, T. L. Williams, X. F. Wang, G. Wu, X. H.

Chen, and J. E. Hoffman, Phys. Rev. Lett. 102, 097002 (2009).[8] D. Hsieh, Y. Xia, L. Wray, D. Qian, K. Gomes, A. Yazdani,

G. F. Chen, J. L. Luo, N. L. Wang, and M. Z. Hasan,arXiv:0812.2289.

[9] F. Massee, Y. Huang, R. Huisman, S. de Jong, J. B. Goedkoop,and M. S. Golden, Phys. Rev. B 79, 220517(R) (2009).

[10] V. B. Nascimento, A. Li, D. R. Jayasundara, Y. Xuan, J. O’Neal,S. Pan, T. Y. Chien, B Hu, X. B. He, G. Li, A. S. Sefat,M. A. McGuire, B. C. Sales, D. Mandrus, M. H. Pan, J. Zhang,

R. Jin, and E. W. Plummer, Phys. Rev. Lett. 103, 076104(2009).

[11] F. C. Niestemski, V. B. Nascimento, B. Hu, W. Plummer,J. Gillett, S. Sebastian, Z. Wang, and V. Madhavan,arXiv:0906.2761.

[12] F. Massee, S. de Jong, Y. Huang, J. Kaas, E. van Heumen,J. B. Goedkoop, and M. S. Golden, Phys. Rev. B 80, 140507(R)(2009).

[13] T. Chuang, M. Allan, J. Lee, Y. Xie, N. Ni, S. Bud’ko,G. Boebinger, P. Canfield, and J. Davis, Science 327, 181(2010).

[14] H. Zhang, J. Dai, Y. Zhang, D. Qu, H. Ji, G. Wu, X. F. Wang,X. H. Chen, B. Wang, C. Zeng, J. Yang, and J. G. Hou, Phys.Rev. B 81, 104520 (2010).

[15] T. Nishizaki, Y. Nakajima, T. Tamegai, and N. Kobayashi, J.Phys. Soc. Jpn. 80, 014710 (2011).

[16] L. Shan, Y. Wang, B. Shen, B. Zeng, Y. Huang, A. Li, D. Wang,H. Yang, C. Ren, Q. Wang, S. Pan, and H. Wen, Nat. Phys. 7,325 (2011).

134520-5

ANG LI et al. PHYSICAL REVIEW B 99, 134520 (2019)

[17] M. L. Teague, G. K. Drayna, G. P. Lockhart, P. Cheng, B. Shen,H.-H. Wen, and N.-C. Yeh, Phys. Rev. Lett. 106, 087004 (2011).

[18] G. Li, X. He, J. Zhang, R. Jin, A. S. Sefat, M. A. McGuire,D. G. Mandrus, B. C. Sales, and E. W. Plummer, Phys. Rev. B86, 060512(R) (2012).

[19] K. Koepernik, S. Johnston, E. van Heumen, Y. Huang, J. Kaas,J. B. Goedkoop, M. S. Golden, and J. van den Brink, Phys. Rev.Lett. 109, 127001 (2012).

[20] Z. Wang, H. Yang, D. Fang, B. Shen, Q. Wang, L. Shan, C.Zhang, P. Dai, and H. Wen, Nat. Phys. 9, 42 (2013).

[21] I. Zeljkovic, D. Huang, C.-L. Song, B. Lv, C.-W. Chu, and J. E.Hoffman, Phys. Rev. B 87, 201108(R) (2013).

[22] C.-L. Song, Y. Yin, M. Zech, T. Williams, M. M. Yee, G.-F.Chen, J.-L. Luo, N.-L. Wang, E.W. Hudson, and J. E. Hoffman,Phys. Rev. B 87, 214519 (2013).

[23] G. Li, L. Liang, Q. Li, M. Pan, V. B. Nascimento, X. He, A. B.Karki, V. Meunier, R. Jin, J. Zhang, and E.W. Plummer, Phys.Rev. Lett. 112, 077205 (2014).

[24] A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, D. J. Singh, andD. Mandrus, Phys. Rev. Lett. 101, 117004 (2008).

[25] C. Zhang, M. Wang, H. Luo, M. Wang, M. Liu, J. Zhao, D. L.Abernathy, T. A. Maier, K. Marty, M. D. Lumsden, S. Chi, S.Chang, J. A. Rodriguez-Rivera, J. W. Lynn, T. Xiang, J. Hu, andP. Dai, Sci. Rep. 1, 115 (2011).

[26] B. Lv, L. Deng, M. Gooch, F. Wei, Y. Sun, J. K. Meen, Y. Xue,B. Lorenz, and C.-W. Chu, Proc. Natl. Acad. Sci. USA 108,15705 (2011).

[27] S. Ran, S. L. Bud’ko, D. K. Pratt, A. Kreyssig, M. G. Kim, M. J.Kramer, D. H. Ryan, W. N. Rowan-Weetaluktuk, Y. Furukawa,B. Roy, A. I. Goldman, and P. C. Canfield, Phys. Rev. B 83,144517 (2011).

[28] M. Rotter, M. Tegel, D. Johrendt, I. Schellenberg, W. Hermes,and R. Pöttgen, Phys. Rev. B 78, 020503(R) (2008).

[29] N. Ni, M. E. Tillman, J.-Q. Yan, A. Kracher, S. T. Hannahs,S. L. Bud’ko, and P. C. Canfield, Phys. Rev. B 78, 214515(2008).

[30] I. Shein and A. Ivanovskii, J. Exp. Theor. Phys. 88, 107(2009).

[31] M. Gao, F. Ma, Z.-Y. Lu, and T. Xiang, Phys. Rev. B 81, 193409(2010).

[32] J.-X. Yin, Ang Li, X.-X. Wu, Jian Li, Zheng Wu, J.-H. Wang,C.-S. Ting, P.-H. Hor, X. J. Liang, C. L. Zhang, P. C. Dai, X. C.Wang, C. Q. Jin, G. F. Chen, J. P. Hu, Z. -Q. Wang, H. Ding,and S. H. Pan, arXiv:1602.04949.

[33] A. V. Balatsky, I. Vekhter, and J.-X. Zhu, Rev. Mod. Phys. 78,373 (2006).

[34] Z. Yin, K. Haule, and G. Kotliar, Nat. Mater. 10, 932 (2011).[35] C. Zhang, L. W. Harriger, Z. Yin, W. Lv, M. Wang, G. Tan,

Y. Song, D. L. Abernathy, W. Tian, T. Egami, K. Haule, G.Kotliar, and P. Dai, Phys. Rev. Lett. 112, 217202 (2014).

[36] Y. Huang, R. Pierre, J. Wang, X. Wang, X. Shi, N. Xu, Z. Wu,A. Li, J. Yin, T. Qian, B. Lv, C. Chu, S. Pan, M. Shi, and H.Ding, Chin. Phys. Lett. 30, 017402 (2013).

[37] J-X. Yin, Zheng Wu, J-H. Wang, Z-Y. Ye, Jing Gong, X-Y. Hou,Lei Shan, Ang Li, X-J. Liang, X-X. Wu, Jian Li, C-S. Ting, Z-Q.Wang, J-P. Hu, P-H. Hor, H. Ding, and S. H. Pan, Nat. Phys. 11,543 (2015).

[38] J.-X. Yin, S. S. Zhang, H. Li, K. Jiang, G. Chang, B. Zhang, B.Lian, C. Xiang, I. Belopolski, H. Zheng, T. A. Cochran, S.-Y.Xu, G. Bian, K. Liu, T.-R. Chang, H. Lin, Z.-Y. Lu, Z. Wang,S. Jia, W. Wang, and M. Z. Hasan, Nature (London) 562, 91(2018).

134520-6